This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present invention, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
In healthy individuals, homeostatic control mechanisms ensure that a balance between fluid gain and fluid loss is maintained. Therefore, maintaining fluid balance is typically not an issue requiring attention. In ill individuals, however, the maintenance of body fluid balance may be cause for great concern. For example, dehydration or edema may occur if fluid balance is not properly maintained. Dehydration of infants and children suffering from diarrhea and/or vomiting can be life threatening if not recognized and treated promptly. Additionally, individuals with congestive heart failure frequently suffer from edema which must be controlled in order to ensure adequate tissue perfusion and to prevent electrolyte disturbances. Also, over-administration of intravenous fluids in critically ill or surgical patients may result in lung edema, with resulting impairment of gas exchange.
Tissue hydration estimation using a hydration index, such as a ratio of water to the sum of water and protein, provides an accurate, objective and non-invasive way to measure fluid balance. Methods for measuring water in tissue by near-infrared (NIR) spectrophotometry have been described in the art. Previous attempts at using a hydration index, however, have been focused on measuring absorption features of the various tissue constituents. Additionally, the previous attempts have required a minimum of two wavelengths to measure the absorbance contributions of water and protein, and at least a third wavelength to compensate for the effect of tissue scattering. If measurements are made at wavelengths where lipids or other constituents, such as hemoglobin, contribute substantially to the absorption spectrum, a fourth or more wavelengths may be used to measure the hydration index accurately. Each additional wavelength increases cost and complexity of the system.